The present invention relates to a highly durable superconducting ceramic structure. The durability described herein has been achieved by the formation of a 10 to 5,000 nanometer-thick plasma-polymerization film of a silazane bond-containing organosilicon compound on the surface of a superconducting ceramic substrate.
Since 1986, a succession of superconducting ceramics based on layered perovskite structures has been discovered. Such ceramics have superconducting transition temperatures (Tc) above the temperature of liquid nitrogen (77.3K) and may be classified into essentially the following three types: the Y-Ba-Cu-O system (Tc=approximately 90K. Phys. Rev. Lett., 58, 405 (1987), etc.), the Bi-Pb-Sr-Ca-Cu-O system (Tc=approximately 120K, Jpn. J. Appl. Phys., 27, L1041 (1988), etc.), and the Tl-Ba-Ca-Cu-O system (Tc=approximately 120K, Nature, 332, 138 (1988), etc.). While the Y-Ba-Cu-O system is easy to synthesize and has a wide composition range over which superconductivity may appear, it has a low Tc. The Bi-Pb-Sr-Ca-Cu-O system does not contain rare-earth elements and has a high Tc but, it has a narrow composition range which permits superconductivity. Finally, the Tl-Ba-Ca-Cu-O system has a high Tc but, the toxicity of thallium is a matter of concern.
Although these three types have both advantages and disadvantages, each has the outstanding attribute of being extremely valuable from an industrial standpoint since in each case Tc exceeds the temperature of liquid nitrogen. In other words, it now becomes possible to use a liquid nitrogen coolant in place of the liquid helium or liquid hydrogen heretofore used. Not only is this highly advantageous from the standpoint of coolant cost and supply, but one can anticipate the appearance of practical products which, from the standpoints of fabrication or application, would have been impossible for prior superconductors.
Because the superconducting ceramics typified by the preceding 3 types may be fabricated in various forms such as bulk, thin films, and wire materials, a broad range of applications can be envisioned for them. For example, one can contemplate the application of wire materials in energy distribution and storage sectors centered on power transmission lines and coils, etc.; the application of the bulk form in devices which use magnets, magnetic shielding, etc.; and the application of thin films in SQUIDs (superconducting quantum interference devices), Josephson junction-based sensors, computer circuitry, etc. Research directed at practical realization is currently very active.
However, it has become clear that contact with water substantially degrades the properties of these superconducting ceramics. For example, in the Y-Ba-Cu-O system a reduction in critical current has been observed, regardless or form, upon standing in air (1987 International Superconductivity Electronics Conference, Rump Session, etc.) and a reduction in Tc and an increase in resistance have been observed upon immersion in hot water (Denshi Joho Tsushin Gakkai Gijutsu Kenkyu Hokoku [Technology Research Reports of the Institute of Electronics, Information, and Communication Engineers], 88 (22), 31 (1988), etc.). The same phenomena have been reported for the Tl-Ba-Ca-Cu-O system. Moreover, although the degree of the decline is less in the Bi-Pb-Sr-Ca-Cu-O system than in the Y-Ba-Cu-O system (Denshi Joho Tsushin Gakkai Gijutsu Kenkyu Hokoku, 88 (146), 19 (1988)), it is estimated that a decline occurs to the same performance level upon long-term contact with water.
These deterioration phenomena are considered to be the biggest bottleneck to practical realization. For example, when these superconducting ceramics are used in microelectronic products such as Josephson devices, transistors. LSI circuitry, and so forth, there is a substantial risk of contact between the superconductor element and the atmosphere during the fabrication, storage, and product use stages. Moreover, water-wash processes (for example, wet etching, etc.) used during fabrication may also subject the superconductor to water. In addition, since moisture readily condenses during immersion in liquid nitrogen in the product use stage, there exists a high risk of contact with water or water vapor. The same high risk of contact with water or water vapor can be easily envisioned for other spheres of application.
Accordingly, it is clear that the most significant problem in the practical realization of superconducting ceramics is the implementation of some means for inhibiting or suppressing the decline in superconducting properties due to water or water vapor. The following methods have been proposed to date as solutions to this problem:
1) methods which induce surface modification by exposing the surface of the superconducting ceramic to an oxygen plasma, etc., and
2) methods based on the formation of a protective film of a metal or organic compound which can prevent the influx of moisture into the superconducting ceramic.
With regard to method 1), not only can it be difficult to form a film which can permanently intercept moisture, but this can also lead to a deterioration in performance (reduction in current density, etc.) through a partial destruction of the superconducting phase. Method 2) is effective in guaranteeing preliminary or feed conductive paths or channels when a protective metal film is formed on a wire superconducting ceramic. However, in other spheres, for example, application in device wiring, etc., this approach cannot be employed due to the high insulation requirements.
Chemical vapor-deposition of protective organic films under method 2 above (i.e., plasma polymerization of monomeric organic compounds) has been proposed based on the following reasons: films can be formed even from compounds which lack functional groups, pinhole-free films are obtainable, films with thicknesses less than 1 micrometer are readily accessible, adherence between the thin film and substrate is good, the obtained film has a crosslinked structure, and the properties of the film can be adjusted from organic to inorganic according to the conditions. For example, a method for protecting superconductors has been reported in which a plasma-polymerized thin film (monomer=trifluoromethane) is formed on the surfaces of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O superconducting thin films (Denshi Joho Tsushin Gakkai Gijutsu Kenkyu Hokoku, 88 (22), 31 (1988), and ibid., 88 (146), 19 (1988)). It was reported that this stopped the deterioration in superconducting properties due to immersion in water or heating in the atmosphere. Nevertheless, these methods still do not provide a permanent protective effect against these environments and have an unsatisfactory protective effect against water vapor.
The present inventor carried out extensive research directed at solving the problems described above and discovered that the formation of a plasma-polymerized film from specific organosilicon compounds on the surface of a superconducting ceramic substrate leads to stable superconducting properties without the problems listed above.